Chronic calcineurin inhibitors (CNIs) are toxic to the kidney and cause vascular endothelial dysfunction. Maintenance of kidney function and avoidance of long-term vascular complications are a major challenge especially in pediatric patients treated with CNIs. The key to reducing the damage caused by immunosuppressant toxicity is early detection. Once detected, the damage caused by immunosuppressant toxicity can be reduced by modification/ individualization of the immunosuppressive drug regimen. However, both the diagnostic tools for early detection and the monitoring tools to guide such an individualization of immunosuppressive drug regimens are currently unavailable and clinical practice still relies on relatively late and insensitive markers such as creatinine in serum and on invasive methods such as percutaneous kidney biopsies. We hypothesize that - the negative effects of CNIs on kidney cell and vascular endothelial cell metabolism in pediatric patients with nephrotic syndrome and after kidney transplantation are reflected by changes in plasma and urine metabolite patterns, and - plasma and urine metabolite markers have better sensitivity and specificity compared to markers currently established in the clinic such as creatinine in serum. We will use a unique comprehensive strategy based on metabolite and protein molecular markers established in the investigators' laboratories to assess the toxicodynamic mechanisms of the CNI tacrolimus in pediatric patients. This proposal will be based on two clinical trials: one based on pediatric patients with nephrotic syndrome that require treatment with CNIs (Aim 1, learning data set) and the other based on pediatric patients after kidney transplantation who receive an immunosuppressive drug regimen based on the CNI tacrolimus (Aim 2, proof-of-concept qualification study in a more complex study population). Among others, these studies will answer the following questions: - Are plasma and urine metabolite and protein pattern changes in pediatric patients with nephrotic syndrome sensitive enough to reflect the negative effects of CNIs on the metabolism and function of the proximal tubule and vascular endothelial cells already after the first dose? - Do changes of urinary metabolite and protein patterns reflect CNI-induced mitochondrial dysfunction? - Can children who are more sensitive to CNI nephrotoxicity and vascular endothelial dysfunction already be identified within the first week of exposure and before any noticeable changes in serum creatinine occur? - Will plasma and urine metabolite and protein pattern changes also be a valid diagnostic tool in the more complex kidney transplant children? - Will these metabolite and protein pattern changes be sufficiently specific to distinguish immunosuppressant toxicity from immune and inflammatory kidney injury? - Will plasma and urine metabolite and protein pattern changes be more sensitive and thus predict the increase of less sensitive markers such as creatinine in serum? If successful, our results will not only provide mechanistic insights into CNI renal and vascular toxicity in vivo but will also provide proof-of-concept for the development of candidate markers into clinical diagnostic tools. Utilization of these markers may allow for (A) predicting tolerability of an immunosuppressive drug regimen and individualization of immunosuppressive therapy, (B) monitoring allograft function and CNI toxicity, and (C) improving clinical long-term outcomes.